md3000i and snapshots?

Nick_Parrott at Nick_Parrott at
Tue Jul 29 09:02:16 CDT 2008

<<still sending this even though Harald has covered this..>>

Hi Scott,

I'm a little busy today with Windows customers :-( so please excuse the
limited answers.. this is really more of a phone call job.

Snapshot metadata is stored on the free space that the disks offer, so
let's show a config for your box;

6 disks, 5 in a R5 and 1 HSP.

RAID 5 of 5 disks = ~1200Gb. If you create a RAID 5 of this size, there
will be nowhere in this disk group (a collection of disks) to store
snapshot meta-data. For this reason, let's opt for an 800Gb RAID 5,
leaving ~400Gb free space in the group. This essentially means you can
create a snapshot of the source VD, and have half the data (~400Gb)
modified on the source VD before you get space issues with the
snapshot's repository space. The SS VD will be presented to the OS as
another VD, however with a numerial suffix, ie: a VD called "BobHoskins"
gets snapped, and is called "BobHoskins1" and so on..)

The SS VD appears to the OS as an entire LUN, however the data consumed
on physical disks is simply the copy of "original" data from the source
VD that needs to be maintained. Anything on the source VD that stays the
same will remain there, and the SS engine will only make a copy of such
data if the source VD is being modified. "Copy-on-write" is the snapshot

When going through the snapshot creation (from wizard or CLI command)
you have an option to set the threshold at which you are alerted to the
repository size, so consider using this to know when the snapshot
meta-data is getting to a particular size.



-----Original Message-----
From: Scott R. Ehrlich [mailto:scott at MIT.EDU] 
Sent: 29 July 2008 12:10
To: Parrott, Nick
Cc: linux-poweredge-Lists
Subject: RE: md3000i and snapshots?

Hi Nick:

I appreciate that feedback, but to get an additional head-start on this 
before reading the referenced URLs, how do I really _know_ how much to 
allocate?   I want to make as much capacity available as possible for 
users.  If I have 6 x 300 GB drives + 1 hot spare, how many disks should
create in a virtual group if I want a RAID 5 set?   Would I use all 6,
then SS will claim no space is available for it to use?   Do I use 5
for the RAID 5, and the 6th for SS?   Maybe someone wants their own 
project set, thus I'd have multiple RAID 5 volumes, or a RAID 5 volume
a RAID 1.

I need some good, specific documentation that details how your SS 
implementation works, so I can truly appreciate how to properly set it
on my end.  Maybe the URLs give that info.  I'll find out later today.

How does Dell's snapshot technology actually work?   I know it takes a 
live moment-in-time image, but what is that image actually based on? Is 
the data compressed?   Does it perform a incremental or date difference
something else method?   Are they full duplicate data, or some method of

pointers to data living somewhere else, since data does change.

I played around with building a couple RAIDs yesterday, but seeing how 
long it takes to build the raid volume, I want to get all the accurate 
info up-front, so I hopefully only need to do this one more time, and at

that time, do it right.

Beyond this email, for the money invested in several servers and the 
3000i, I should also look to my local dell reps for additional inside 
technical support and possibly more step-by-step procedures to get me to

the end goal as quickly as possible.

This list is fantastic, which is actually why I use it first.

Thanks again for any insights you, and everyone else on the list, can 


On Tue, 29 Jul 2008, Nick_Parrott at wrote:

> Scott,
> Snapshots are by default (in simple mode) stored in the disk group
> remaining space, so when creating a disk group and then VD, consider
> amount of space required for snapshot changes, a threshold can be set,
> so that when the SS VD reaches say, 50% of the original VD, you'll be
> alerted.
> It's also possible using the advanced mode in the snapshot wizard to
> specify another disk group for storing snapshot meta-data.
> This doc page may assist; (for 3000i however is same for both
> 3000/3000i)
> apshot.htm#wp1001413
> You'll have to create the VD first before you look to configure
> snapshots. There is also a CLI tool for the host that enables you to
> script such procedures;
> ndex.htm
> napshot.htm#wp1074424
> Regards,
> Nick
> -----Original Message-----
> From: linux-poweredge-bounces at
> [mailto:linux-poweredge-bounces at] On Behalf Of Scott R.
> Sent: 28 July 2008 22:28
> To: linux-poweredge-Lists
> Subject: md3000i and snapshots?
> I have a new md3000i connected to a new PE2950 running CentOS 5.0
> 64-bit,
> out of box.
> I'm talking to the md3000i find now, and am learning how the
> configuration
> manager works.  I've enabled snapshot capability that we've paid for.
> What would be _really_ nice, that I don't see in the March 2008 CD I
> have
> for installation of the configuration manager, is the magical
> incorporation of snapshot allocation into the virtual disk building
> process.   If I select drives for logical volumes, I would hope there
> would either be allocated space for snapshoting afterwords, or that
> feature would be part of the configuration/build process.
> It can take several hours to build an array, only to realize that I
> didn't
> leave enough space.   With the design of Dell's GUI management tool,
> least I could ask is for as seemless a configuration as possible.
> Now I have to take a lot of extra time to figure out if I want one
> massive
> RAID 5, and if so, how many disks, then wonder/hope I leave enough
> for snapshotting.  If not, delete the RAID, hours again rebuild a
> smaller
> one, then create the snapshot stuff.
> Is there an easier way to manage/incorporate snapshotting with RAID
> volume
> creations?
> Snapshotting is why I went with the 3000i to start with instead of the
> much more expensive netapp.  I expect to get the most out of it, if I
> can
> figure out an easy to get to it...
> Thanks.
> Scott
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